For the past 60 years, influenza vaccination has been clinically used to prevent influenza infection. However, a significant obstacle to effective vaccination is the occurrence of vaccine nonresponses, which occur more frequently in settings such as aging and chronic HIV infection. Unfortunately, these populations are typically also the ones most at risk for morbidity and mortality from infection. The primary immune correlate of protection after influenza vaccination is neutralizing antibody production by B cells, which require help from T follicular helper (Tfh) CD4 T cells. We have identified a population of circulating Tfh cells (Tfh-like) that are highly similar to lymphoid Tfh based on phenotypic, transcriptional, and functional assays. At baseline, the elderly had a reduced circulating frequency, greater activation, and diminished functional ability in Tfh-like cells as compared to young adults. After vaccination, the Tfh-like response correlated with antibody production in the young but was blunted in the elderly. Considerable evidence suggests a strong relationship between inflammation and senescence and may lead to accelerated senescence in the context of HIV infection. We hypothesize that the effects of chronic, excessive inflammation on Tfh-like cells predict senescence and poor antibody responses to vaccination in the setting of aging and HIV infection. The specific aims to be addressed in this project are: Aim 1: Is chronic inflammation linked to senescence of Tfh-like cells? We have identified immune activation and senescence in Tfh-like cells but it is unknown if systemic inflammation plays a role. I will test whether serum inflammatory markers predict a comprehensive set of senescence changes in Tfh- like cells from young and elderly adults, followed by correlation to the influenza vaccine response. Aim 2: Are there senescence-related changes in intracellular signaling pathways in Tfh-like cells? Prior studies suggest age-related alterations in signaling pathways downstream of cytokine receptors. Using CyTOF mass cytometry, I will evaluate the STAT1, STAT3, and STAT5 response to cytokines relevant to Tfh-like cells. This will address whether poor Tfh responses could be due to cell- intrinsic factors. Aim 3: Does uncontrolled HIV viremia lead to early senescence in Tfh-like cells? HIV infection is thought to cause accelerated senescence which may be due to increased inflammation. Using a previously generated cohort of influenza-vaccinated subjects, I will test whether active HIV viremia and serum markers of inflammation predict diminished vaccine responses and whether excess senescence is manifested in Tfh-like cells when compared to HIV-aviremic and HIV-uninfected subjects. Together, these data will elucidate the relationship between inflammation and senescence in Tfh-like cells. Ultimately, the goal is create opportunities for better vaccines in those with evidence of inflammation and senescence.